Data accompanying "Beneficial use of dredged sediment reshapes mangrove establishment across species and critical life-history stages"

Manuscript Abstract Introduction: Understanding the mechanisms that limit species establishment and persistence in ecosystems is critical for effective restoration design. Yet it remains unclear how dredged sediment amendments employed to counteract wetland loss affect mangroves’ contribution to coastal resilience, particularly in subtropical ecosystems where mangroves are expanding their geographic range. Objectives: We tested the interactive effect of relative tidal elevation and sediment amendments on Avicennia germinans (black mangrove) and Rhizophora mangle (red mangrove) establishment through passive (propagule establishment) and active (planted seedling) revegetation approaches. Methods: Vegetation surveys informed a series of field experiments manipulating sediment type (dredge versus natural marsh), mangrove species, and life-history stage across elevations. Results: Propagule establishment was unaffected by elevation in both sediment types although success in dredged sediments varied by species: Avicennia establishment was unimpeded while Rhizophora had poor success. Both species had a sediment-dependent relationship with elevation as seedlings. In natural marsh sediments, Avicennia survival and biomass increased with elevation while Rhizophora seedlings followed the opposite pattern. In dredged sediments, both species performed comparably across elevations and exceeded outcomes in the natural marsh with two times higher survival and six times more biomass in Avicennia, and three times higher survival and biomass in Rhizophora. Conclusions: Mangrove establishment was unaffected by elevational changes in dredged sediments, suggesting greater flexibility in setting target restoration elevations. While Avicennia can establish through active or passive approaches in dredged sediment, propagule establishment functions as a bottleneck to Rhizophora reintroduction, thus requiring seedling outplanting to achieve success. Implications for Practice: Successful mangrove reintroduction in wetlands restored with dredged sediment is unlikely to be achieved with a single approach as mangrove establishment success varies with species and life-history stage (propagule versus seedling). Ultimately, we provide a management decision framework to guide restoration practitioners and coastal managers towards the strategy best suited to their restoration objectives in regions where sediment amendments overlap with mangrove dominance. Since sediment addition can promote or inhibit mangrove establishment and persistence, future research should explore how this emerging approach to ecosystem restoration influences long-term habitat coverage in wetlands, particularly in dynamic systems transitioning between saltmarsh and mangrove forest. Methods Field surveys We surveyed nine lower to upper intertidal marsh-mangrove ecotones between May and August 2023 (Figure 1C) to capture the natural distribution of mangroves along an elevation gradient relative to other coastal wetland plants, and to assess how these patterns varied by species and life-history stage. At each site, we established three 36-60m transects traversing the natural elevation gradient, typically perpendicular to the Matanzas River shoreline. Transects were placed at least 15m apart with transect length increased as needed to fully capture changes in plant composition between unvegetated mudflat and upland vegetation. Surface elevation (shown henceforth in cm above mean sea level [cm MSL]) was recorded at 4 m intervals along each transect using a Real-Time Kinematic Global Navigation Satellite System (RTK GNSS). At the same intervals, plant percent cover was visually estimated within 50×50 cm quadrats. Species were grouped into three functional categories: (1) “Spartina,” (2) a “high marsh complex” consisting of Juncus roemerianus (black needlerush), Batis maritima (saltwort), Borrichia frutescens (sea oxeye) and Salicornia (pickleweed), and (3) “mangrove”, including Avicennia and Rhizophora, which were the only mangrove species observed along transect lines. To identify patterns of mangrove arrival, establishment and persistence, we also counted Avicennia and Rhizophora along each transect and distinguished between life-history stages into propagules (young mangroves with the cotyledon or epicotyl evident), seedlings (unbranched trees with no prop roots), or adults (branched trees with [Rhizophora] or without [Avicennia] prop roots and canopy cover >0.25 m2). Propagules and seedlings were counted within the same 50×50 cm quadrats used to determine percent cover. Adult trees, because of their low density and larger size, were counted within 4×4m quadrats placed every 4 m along transects. Propagules were counted again at four of the nine survey sites in October 2023 (during annual propagule dispersal) due to low densities in the initial summer survey, and only these latter October counts are included in our results. Experiment 1: Seedling survival and performance In July 2023, we initiated an experiment to test the interactive effects of tidal elevation and substrate type on the survival and growth of Avicennia and Rhizophora seedlings. The experimental plots were deployed at four mean tidal elevations along the site’s natural gradient. These were chosen based on surveys to represent the vegetation zones that characterize this study area where Spartina dominates the lowest elevation (16 cm), mangrove cover increases with elevation from 27 to 49 cm, and Avicennia dominates at the highest elevations (66 cm; Table 1 shows the standard deviation [SD] associated with each experimental elevation). Natural marsh plots (+0 cm added sediment) were established at each elevation to represent existing marsh conditions while sediment addition plots (+15 or +40 cm; referenced cumulatively as elevated plots) experimentally created higher surface elevations within plot enclosures. At 16 cm MSL, 15 and 40 cm of sediment were added to approximate 27 and 49 cm MSL respectively, while 15 cm of sediment was added at 49 cm MSL to approximate 66 cm MSL (Table 1; Figure 1A). Together this resulted in seven (7) elevation×sediment treatments applied to both mangrove species (n=10 per treatment combination for each species totaling n=140 plots; Figure 2A). Plot boundaries were defined using modified, open-bottomed 5-gallon buckets (30-cm diameter) sunk 5 cm into the marsh surface and cut to the aboveground height of the sediment thickness to be added (+15 cm or +40 cm). Natural sediment plots (+0 cm added) were designated by 7cm-tall rings also sunk into the marsh surface to a depth of 5 cm. Added sediments (90% sand, 10% silt [Hay et al. 2026]) were sourced from a dredged material management area north of the project site and manually added to sediment addition plots over the course of two days. Sediment settling and compression of underlying sediment was accounted for by topping off plots with additional sediment three weeks after installation, and any potential surface water ponding was prevented by drilling 2mm drainage holes through the plot wall at the sediment surface in each plot. Once sediment was added, three Avicennia or Rhizophora seedlings were planted in each plot (Figure 2B) following the removal of any naturally established seedlings. All seedlings were 1-2 years old at the time of collection (mean height=38 ± 6 cm SD) and sourced locally. Seedling survivorship was recorded 14 months post-planting when seedlings with no green leaves were recorded as dead while all other seedlings were recorded as alive. Surviving seedlings were also harvested and dried at 60oC to a constant mass to determine the total aboveground biomass in each plot which was used as a proxy for seedling performance. Experiment 2: Propagule establishment To complement the above experiment which focused on seedling survival and growth, we conducted another experiment to isolate the effects of tidal elevation and sediment type on mangrove propagule establishment in October 2024. We had three treatments at starting elevations 16 and 49 cm MSL: natural marsh plots (+0 cm added) that reflected marsh conditions, plots excavated to 8cm-depth and backfilled to ground surface elevation with dredged material (hereafter referenced as “backfilled plots”) and sediment addition or elevated plots with 15 cm of dredged material, resulting in six elevation×treatment combinations (n=6 per treatment combination for each species for a total of n=72 plots; Figure 2C). Locally sourced black (n=12 per plot) and red (n=3 per plot) propagules were placed on the plot surface and wire mesh enclosures (7 mm mesh size) were added to prevent both experimental propagules from floating beyond the plot boundary and other propagules from floating in (Figure 2D) as the experiment was established to test establishment rather than arrival or retention. One year later when all propagules had either successfully established and transitioned to the seedling stage or died, we recorded the number of seedlings in each plot. Sediment properties To assess soil properties in the various treatments, syringe cores (5 cm depth, 2.7 cm diameter) were collected at the conclusion of the propagule experiment from the center of each plot including plots from the seedling experiment which had not yet been dismantled (n=6 per treatment). Cores were dried at 60oC to a constant weight to assess relative differences in bulk density (soil dry weight in a known volume) and moisture (% water loss of sample) and subsequently burned for 3 hours at 650oC to determine organic matter (OM) content. Data description The data file contains all datatsets used in our manuscript, organized across ten excel sheets: survey (1) sites, (2) percent cover, (3) propagule counts, (4) seedling counts, and (5) adult counts. We also include (6) survival data, and (7) aboveground biomass data from the seedling experiment, (8) propagule establishment data from the propagule experiment, and (9) soil properties from plots used in both experiments. A detailed description of each sheet and their associated variables are provided in an additional "README" sheet (10) in the file.